U.S. Pat. No. 5,850,409 discloses feeding back measurement signals, converted by means of a monitor diode, for regulating the laser power and the output amplitude of the modulated optical data. In this case, a pilot tone is superposed on a data signal, said pilot tone having a relatively low frequency and a low amplitude compared with the data signal. The output amplitude of the modulated data is regulated to a constant value. For this purpose, the amplitude of the pilot tone is set to a constant fraction of the modulation current so that the optical output power is a constant fraction of the amplitude of the optical data with the frequency of the pilot tone. The modulation current is regulated by means of a feedback, so that the optical amplitude of the transmitted data can also be regulated to a desired value.
A second feedback loop is used to regulate the average optical output power of the laser to a fixed value. For this purpose, IBIAS is regulated in such a way that the average optical power P0 is constant independently of the laser slope and thus temperature or aging. Both regulating principles together ensure a constant extinction ratio ER.
However, this regulating principle has the following disadvantages for directly modulated lasers at high data rates:
1. In order that the laser achieves a sufficient modulation bandwidth, IBIAS-Ith, i.e. the difference between the bias current and the threshold current of the laser, has to be chosen to be relatively high, for example 30 to 50 mA, thereby already resulting in relatively high laser currents. With increasing temperature and aging, the laser slope decreases and the threshold current rises. In the case of regulation to constant power, the reduced laser slope has to be compensated for by a significant increase in the laser current.
This may very rapidly lead to the maximum available or permissible laser current being exceeded. Moreover, a very large modulation amplitude Imod is then necessary for a constant extinction ratio, which in turn is difficult to achieve at high data rates and furthermore leads to undesirable relaxation oscillations at the high level “1”.
These relationships are illustrated in FIG. 3, in which the power P0 of a laser diode is plotted as a function of the difference IBIAS-Ith. Smax specifies the maximum slope of the laser diode at the beginning of life (BOL) and at low temperature (25° C.), and 5 min specifies the minimum slope of the laser diode at the end of life (EOL) and at high temperature (75° C.). The modulation current of a data signal is specified by Imod.
On the one hand, it can be seen directly from FIG. 3 that, in the case of a control to P0=constant, a decreasing laser slope leads to a lower extinction ratio ET=Pmax/Pmin. Although this can be compensated for by increasing the modulation current Imod, an aging-dictated change in the extinction ratio ER is generally unavoidable. The consequence of this, however, is that a very high value of ER must initially be present at the beginning of life (BOL) in order to be able to still guarantee the required ER at the end of life (EOL). However, a high extinction ratio disadvantageously leads to a high degree of relaxation oscillations of the laser.
On the other hand, FIG. 3 reveals that as the laser slope decreases, for a regulation to constant power P0 it is necessary for the laser current to be considerably increased.
2. The regulation to constant power is relatively inaccurate since the ratio of detector current of the monitor diode to the power in the fiber changes with respect to temperature and to aging. This applies especially to the DFB laser modules that are usually used for high data rates and the customary construction in which the monitor diode measures the radiation emerging from the rear facet. This has the effect that the slope has to be complied with extremely accurately during the production of the laser modules, in particular the fiber coupling.
Moreover, very narrow tolerances have to be complied with for a laser and a regulator in order to enable the operation of a laser within predetermined specification limits such as maximum and minimum optical power, maximum BIAS current, temperature range, extinction ratio and frequency response.